It couldn’t be any easier! Follow these steps to setup your Exchange account on your iPhone.
Go to Settings -> Mail -> Add Account
Enter your Email Address.
Enter your Domain\Username (this is tricky! contact TMG for your domain).
Enter your Password
Hit Next, wait a few seconds while the iPhone attempts to make a connection to your Exchange server. If the attempt fails, the iPhone will ask for your Exchange Server Address
Enter your Exchange Server Address (usually rww.yourdomain.com)
Select what you would like to sync and hit Save.
Now go to your mail account and open to view your folders! Simple! All of your email is PUSHED to your iPhone, try it! Send an email from your Yahoo mail account to your Corporate Exchange account and watch the email push to your iPhone!